WASHINGTON, Feb 25 (Reuters) - The Pentagon said on Monday
an F-35 test plane was involved in an incident on Feb. 14 that
caused smoke in the cockpit, and it was sending the affected
parts back to their manufacturer, Honeywell International Inc
, for a detailed inspection.

Kyra Hawn, spokeswoman for the $396 billion F-35 Joint
Strike Fighter program, said an initial assessment of the
incident at a Maryland air base showed it was isolated,
software-related, and posed minimal risk. The Pentagon has made
temporary changes to prevent another smoke incident, she said.

News of the previously unreported incident comes just days
after U.S. military officials grounded the entire fleet of
Lockheed Martin Corp F-35 jets for the second time this
year after discovering a 0.6 inch crack on a fan blade in the
single jet of another test plane.

A spokesman for enginemaker Pratt & Whitney, a unit of
United Technologies Corp, said the blade assembly
arrived at the company's Middletown, Connecticut, facility on
Sunday evening and engineering teams were examining it now.

Honeywell builds the plane's "power thermal management
system," which uses a lithium-ion battery similar to those whose
failures have grounded Boeing Co's entire fleet of 787
airliners, but Hawn said there was no connection
between the Feb. 14 incident and the F-35's lithium-ion
batteries.

"It has no linkage whatsoever with the lithium-ion
batteries," Hawn said. She said the Feb. 14 incident was the
only one involving smoke in the cockpit of an F-35 "in recent
program history."

Lockheed is building three models of the new radar-evading
warplane to replace nearly a dozen fighter jets in use by the
U.S. military and its allies. The Pentagon plans to buy 2,447 of
the advanced fighter in coming decades.

Honeywell said it would inspect the system, which manages
the distribution of hot and cold air in the F-35 fuselage, once
it arrived at the company's Phoenix testing facility.

SOFTWARE ISSUE

The U.S. Federal Aviation Administration grounded Boeing's
787 commercial airliner on Jan. 16 after two separate battery
failures, including one that triggered an emergency landing in
Japan after the crew detected smoke in the cockpit.

Boeing's biggest rival, Airbus, a unit of Europe's EADS
, has decided in the aftermath to skip using lithium-ion
batteries in its new A350 airliner.

But the Pentagon earlier this month said it would continue
using lithium-ion batteries on the F-35 since they were made by
different manufacturers from those used on the 787, and had been
found to be safe after extensive testing.

Hawn said an initial assessment of the Feb. 14 incident
involving BF-2, one of the Marine Corps' short takeoff, vertical
landing variants, had linked the problem to a software issue,
not a problem with the hardware on the auxiliary power unit.

The entire temperature management system was being sent to
Honeywell for a closer inspection and development of a permanent
fix, she said, noting that the plane was going through
developmental testing specifically to find any such problems.

"This is the purpose of test, development and initial
training in any program - identify discrepancies, develop fixes,
and put them in place to ensure safety of operations," she said,
adding that initial assessment indicated "minimal risk and (a)
relatively uncomplicated resolution."

Honeywell spokesman Nathan Drevna said the company would
inspect the system once it arrived at the Phoenix facility.

"The pilot landed safely. The Honeywell-related products are
being shipped to our testing facility so we can quickly inspect
and determine next steps with our customer," Drevna said.

SMOKE BUT NO FIRE

Lockheed spokesman Michael Rein said there was no sign that
a lithium-ion battery was involved, and the battery had not been
pulled from the F-35 for further review. "There is no evidence
that the lithium ion batteries are a contributor to this event,"
he said, adding, "no battery faults were observed at any time."

One U.S. defense official familiar with the incident said
the F-35 pilot reported "trace amounts of smoke" in the cockpit
after he followed procedures to stop and restart the auxiliary
power unit when a caution light came on.

The pilot then halted the test flight and landed safely at
the base, without ever declaring an in-flight emergency, said
the official, who was not authorized to speak publicly, adding,
"there wasn't any fire associated with the smoke incident."

Procedures have now been changed so that pilots do not
restart the backup power unit in flight, the official said.

Honeywell's Drevna said the temperature control unit is part
of a bigger integrated power package (IPP), also built by
Honeywell, which uses a 270-volt lithium-ion battery to start
the engine, and also provide emergency backup power. Only the
temperature control system was being sent back to Honeywell.

Lockheed said the power and thermal system was not using the
battery at the time of the Feb. 14 incident and the battery
checked out as fully functional during a post-flight review. The
IPP also functioned as designed, he said.

A malfunctioning valve in the larger IPP system grounded the
F-35 for two weeks in August 2011, but this was a separate
issue, the Pentagon said on Monday.